Purpose/Function:

Runoff first passes through multiple pretreatment mechanisms to trap sediment and organic matter before it reaches the practice. As the stormwater penetrates the underlying soil, chemical and physical adsorption processes remove pollutants.

Initiation protocol:

Design specifications require infiltration basins and trenches to be built in good soil, they are not constructed on poor soils, such as C and D soil types. Engineers are required to test the soil before approved to build is issued.

Implementation Factors (level of difficulty):

Difficult. Engineers need to properly design and inspect this practice.

Funding Sources / Options:

Many urban centers initiate a Stormwater Utility Program to pay for this practice.

Costs:

Costs estimated as $ per impervious acre treated.

Cost Estimates

EPA

King & Hagan

Average

Initial

$13,433

$15,863

$14,648

Annual

$548

$217

$382

Lifespan (yrs)

20

20

20

Annualized

$1,219.65

$1,010.15

$1,114.90

Notes: King & Hagan figures are expressed in units of “impervious acres treated”. To convert to urban acres treated, this analysis uses a factor of 0.25, based on two assumptions: (1) there are three acres of pervious land for every acre of impervious and (2) the additional volume required for pervious treatment is minimal.

Load Reduction Efficiency:

Total Nitrogen removed per impervious acre treated per year

Low

4.08

Medium

6.21 lbs.

High

15.27 lbs.

Cost per pound removed = between $73 and $273

Total Phosphorous removed per impervious acre treated per year

Low

0.38 lbs.

Medium

0.50 lbs.

High

1.00 lbs.

Cost per pound removed = between $1,114 and $2,955

Total Suspended Solids removed per impervious acre treated per year

Low

22 lbs.

Medium

139 lbs.

High

543 lbs.

Cost per pound removed = between $2.05 and $50.00

Operation & Maintenance:

To receive credit over the longer term, jurisdictions must conduct yearly inspections to determine if the basin or trench is still infiltrating runoff.

Planning Questions to Consider:

Infiltration practices have the greatest runoff reduction capability of any stormwater practice and are suitable for use in residential and other urban areas where measured soil permeability rates exceed half inch per hour. To prevent possible groundwater contamination, infiltration should not be utilized at sites designated as stormwater hotspots.

Technical Notes:

The major design goal for infiltration is to maximize runoff volume reduction and nutrient removal.